The interrelationships between rods and cones during visual adaptation will be examined by psychophysical experiments in the near peripheral retina. Since our analysis is based upon the Stiles two-color increment threshold technique, a central assumption is that threshold is always mediated by the most sensitive spectrally tuned (receptor related) mechanism. By using the Stiles Crawford effect and by determining action spectra, this assumption is critically tested. The hypothesis that spatial parameters determine whether rods and cones are independent or interact will be examined in situations involving increment thresholds, long-term dark adaptation, and short-term (Crawford, 1947) adaptation. Several experiments will be performed with color-defective subjects in order to assess the types of cone mechanisms which interact with rods.